A recent analysis of five studies published in the International Journal of Gynecology & Obstetrics found that vitamin D levels below 20 ng/ml were associated with an increased risk of miscarriage (spontaneous pregnancy loss or SPL) in the first trimester.
Spontaneous pregnancy loss, also known as a miscarriage, is the most common type of pregnancy loss. It usually occurs due to the improper development of the fetus. As many as half of all pregnancies end in miscarriage, most commonly before a woman misses her period or even knows she is pregnant. Approximately 15-25% of recognized pregnancies end in miscarriage. More than 80% of miscarriages occur within the first three months of pregnancy.
Research shows that vitamin D likely plays a critical role in a healthy pregnancy. A randomized controlled trial, the highest quality study design, found that women who supplemented with 4,000 IU of vitamin D daily during pregnancy were more likely to have an uncomplicated birth. Whereas, women taking less vitamin D were more likely to have preterm babies, develop gestational diabetes, preeclampsia or infections.
The vitamin D receptor and the enzyme used to convert vitamin D to active vitamin D are expressed in reproductive tissues, including the human placenta, endometrium and ovaries. Thus, it appears that vitamin D plays an important role in the regulation of reproductive processes. Furthermore, studies have demonstrated that vitamin D deficiency has detrimental effects on critical stages of pregnancy, such as placental development and embryo implantation. Therefore, researchers recently conducted a meta-analysis to test their hypothesis that vitamin D deficiency puts mothers at an increased risk for SPL (miscarriages).
A meta-analysis is the statistical method for combining data from multiple studies. Clinical implications and the validity of a hypothesis cannot be based on the results from a single study, because results often vary from one study to another. A meta-analysis enables researchers to provide a more definitive conclusion, because it summarizes the findings from many studies.
In this meta-analysis, the researchers pulled data from five studies with a total of 10,630 pregnant women. All studies followed a case-control or cohort design, contained a clear definition of vitamin D deficiency and sufficiency, a definition of miscarriage and reported the prevalence of miscarriage in different pregnancy weeks or calculated the risk of miscarriage based on vitamin D status.
The researchers found that pregnant women with vitamin D levels of less than 30 ng/ml were not at an increased risk of miscarriage compared to women with vitamin D levels of 30 ng/ml or more. The same was true when they compared the incidence of miscarriage between women with vitamin D levels of 20 ng/ml or higher to below 20 ng/ml.
However, when the researchers specifically looked at the incidence of miscarriage in the first trimester, which included two studies, they found that women with vitamin D levels below 20 ng/ml were 2.24 times more likely to experience a miscarriage (SPL) compared to women with vitamin D levels of 20 ng/ml or greater.
The researchers concluded,
“The present findings imply that the maternal vitamin D level might be related to SPL in the first trimester, but not in the second, with a serum 25(OH)D concentration of less than 20 ng/mL increasing the risk of early SPL by a factor of 2.24.”
They went on to state,
“This finding indicates that severe maternal vitamin D deficiency may be a modifiable risk factor in early embryonic development. Therefore, it is essential that maternity and child care institutions highlight the need for vitamin D supplementation in early pregnancy.”
It’s important to keep in mind that this meta-analysis only included five studies, a relatively small number. In addition, none of the studies were clinical trials, meaning that they were unable to prove that vitamin D directly prevents miscarriages. Therefore, larger studies are needed to evaluate the use of physiological doses of vitamin D supplements for the prevention of miscarriages.
Tovey, A. & Cannell, JJ. The relationship between vitamin D and miscarriages. The Vitamin D Council Blog & Newsletter, May 30, 2017.